Gas turbine engine efficiency is closely related to operating temperatures and the continuing search for increased efficiency has been satisfied in part by substantial increases in gas turbine engine operating temperatures. Turbine engine internal gas temperatures now routinely exceed 2700.degree. F. Since the melting point of commonly used superalloys is only about 2400.degree. F., this high temperature operation is made possible only through the use of internal cooling. However even with internal cooling, hardware operating temperatures are reaching the material melting points. Additionally, the use of internal cooling detracts from engine efficiency in that the cooling air which originates in the compressor, which has some energy content is wasted air which could otherwise be used to support combustion. For these reasons ceramic thermal barrier coatings have been developed and are finding increasing applications. Such coatings were first used in the combustor section of gas turbine engines, but are now used in other applications.
Ceramic thermal barrier coatings are usually applied over a bond coat which is a highly oxidation resistant material such as an MCrAlY (see for example U.S. Pat. Nos. 3,542,530, 3,676,085, 3,754,903 and 3,928,026), or other oxidation resistant alloy as described in U.S. Pat. No. 4,371,570, column 3 lines 5-20. The ceramic coating is generally based on zirconia which is stabilized with additions of magnesia, yttria or other additives. Other ceramic materials such as alumina have also been proposed for ceramic coatings. The ceramic material is most often applied by plasma spraying (see U.S. Pat. No. 4,055,705, but may also be applied by vapor deposition as shown for example in U.S. Pat. No. 4,321,311. Both processes produce coatings containing 5-15% porosity, cracks and voids.
Ceramics are hard, durable, abrasion resistant materials and when it is necessary to remove a ceramic coating to rework a defective component during initial production or to refurbish parts after engine operation, it is extremely difficult to remove the ceramic coating without damaging the substrate. Insofar as is known the only practical method for removing ceramic coatings heretofore has been grit blasting in which abrasive particles are blown by compressed air against the ceramic surface to mechanically abrade the coating. This is a manual process since the process must be terminated immediately upon exposure of the substrate. Even in the hands of skilled operators however this is an inexact process which produces excessive amount of scrap parts.
Past attempts to use fluorine to remove ceramic coatings have (to my knowledge) involved the use of liquids containing HF. Aqueous HF is an exceptionally dangerous material which will produce severe burns even in low concentrations and short exposures. Equally significant however is the fact that aqueous solutions do not penetrate the coating but slowly dissolve the coating from the free surface inward. This aqueous dissolution attack produces sludge which must be periodically removed since it inhibits further reaction. Due to these limitations, use of aqueous HF is not a viable method to remove ceramic coatings.
One prior patent (U.S. Pat. No. 2,279,267) has suggested passing HF gas through a retort at room temperature to remove (nonporous vitreous enamel by general attack in a process requiring about 32 hours. The reaction is described as being principally betwen the HF and the SiO.sub.2 enamel constituent. Other references deal with use of HF to descale metal and to clean semiconductor production apparatus. Use of gaseous HF to clear metal surfaces in preparation for brazing is also known.
Accordingly, it is an object of this invention to provide a method for the easy and economic removal of porous ceramic coatings from metallic substrates without significant substrate attack.
It is another object of the invention to provide a method for removing porous ceramic coatings from metallic substrates which will leave the substrates in a clean state free from oxides.